Journal of Sol-Gel Science and Technology 33, 93–97, 2005 c 2005 Springer Science + Business Media, Inc. Manufactured in The United States.
ZrO2 and Cu/ZrO2 Sol–Gel Materials Spectroscopic Characterization ´ ´ T. LOPEZ, M. ALVAREZ AND R. GOMEZ Department of Chemistry, Universidad Aut´onoma Metropolitana-Iztapalapa, P.O. Box 44-534, M´exico, D. F. 09340
[email protected]
D.H. AGUILAR AND P. QUINTANA Department of Applied Physics, Cinvestav-IPN, Unidad M´erida, A.P. 73. Cordemex, 97310, M´erida, Yucat´an, M´exico
Abstract. Copper-doped zirconia (1% mol) and zirconia powders were prepared by the sol–gel process, using zirconium n-butoxide and copper nitrate as precursors. The resulting xerogels are nanocrystalline and exhibit different properties from the corresponding microcrystalline materials. The copper nitrate salt was dissolved and cogelled in situ at the initial stage of the reaction. The properties of the resulting materials were studied by XRD, FTIR and UV-Vis. The as-prepared samples were amorphous and crystallized to the tetragonal zirconia phase at 400◦ C. At temperatures higher than 600◦ C, the monoclinic phase was also obtained. No evidence of discrete crystalline copper compounds was observed, consistent with good dispersion of the dopant. Several bands were observed by FTIR in the 4400–3000 cm−1 region, which diminishes in intensity and shifted to higher wavenumbers with heating. The bandgap energy (E g ) was strongly modulated by the presence of the dopant and heating temperature, with increasing temperature leading to a corresponding decrease in E g . Keywords: copper-zirconia sol–gel, copper-zirconia band gap, copper-zirconia XRD, copper-zirconia FTIR
1.
Introduction
The performance of heterogeneous catalysts depends on their structure and stability [1]. Zirconia exhibits many desirable structural and electronic properties, and has been used in many applications, in particular as a catalyst support. Depending on the preparation method and thermal treatment, monoclinic, tetragonal, cubic, or a mixture of these phases can be obtained, although the tetragonal form generally exhibits better catalytic properties. Zirconia is a suitable support for transition metals such as Cu, and Cu/ZrO2 has so far been prepared by precipitation, coprecipitation or impregnation of preshaped zirconia carriers with appropriate salt solutions [2]. The sol–gel method has attracted considerable attention for the preparation of metallic catalysts, since the constituents are mixed in a molecular scale and gener-
ates a uniform distribution of the metal upon the support [3]. Wombach et al. [4] shown that supported copper is a good catalyst for several reactions, including CO2 hydrogenation and combustion. Sun and Sermon investigated sol–gel derived Cu/ZrO2 catalysts and showed that the dopant can be present in two forms: surface aggregates and dispersed Cu(II) in ZrO2 substitutional sites [5]. This phenomenon generates metal-support and ion-ion interactions and favors the formation of oxygen vacancies, leading to ionic conduction [6]. It is well known that zirconia is an insulating material but sol–gel zirconia and doped zirconia behave as semiconductors with some photochemical properties. In the present work, zirconia was doped in situ with Cu(NO3 )2 . It is believed that most of the copper will be dispersed in the form of nanosized particles on the zirconia’s surface, although a small amount is introduced into the zirconia network [7]. In this work, zirconia and
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copper-doped zirconia (Cu/ZrO2 ) materials were synthesized via the sol–gel method, and characterized by XRD, FTIR, and UV-Vis spectroscopy to evaluate the effect of copper on the structural and electronic properties of zirconia. 2. 2.1.
Experimental Sample Preparation
ZrO2 and Cu/ZrO2 xerogels were prepared by the sol–gel method. Water (6 mL) and tert-butyl alcohol (61 mL; J.T Baker, 99.7%), were mixed with continuous stirring over 10 min. For the Cu/ZrO2 sample, 0.2045 g of Cu(NO3 )2 ·2.5H2 O was dissolved into the water. HNO3 , used as a catalyst, was added to adjust the pH to 3. The solution was heated at 70◦ C for 10 min, cooled to 60◦ C and zirconium (IV) n-butoxide (39 mL; Aldrich 80% in 1-butanol) was added drop-wise to the solution. Refluxing was then maintained until gelation occurred. The resulting samples were dried at 90◦ C, and then annealed in air for 4 h at 200, 400, 600 and 800◦ C, with a heating rate of 5◦ C per min. 2.1.1. X-Ray Diffraction. XRD was used to detect the crystalline phases in the samples. Measurements were carried out using a Siemens model D-5000 diffractometer, with Cu Kα radiation and a graphite monochromator. A step size of 0.03◦ (2θ) and a step time count of 1 s were employed. 2.1.2. FTIR Spectra. The catalysts were characterized by FTIR spectroscopy in a Paragon-1000 spectrophotometer. Samples was mixed with KBr (5% W), and pressed into clear pellets for analysis. 2.1.3. UV-Vis Spectra. UV-Vis (diffuse reflectance) spectra were obtained with a Varian Cary-1 UVVis spectrophotometer, equipped with an integrating sphere. To obtain the spectra, self-supporting pellets were prepared, and all spectra were referenced to a 100% reflectance sample (MgO). The band gap values were calculated by linearization of the slopes in the spectra for each sample using Eq. (1): Eg =
1239 ∗ b −a
(1)
where E g is in eV, and b and a are obtained by a linear fit.
3.
Results and Discussion
The competition between hydrolysis and condensation strongly affects the composition, structural and optical properties of the solids. Typical reactions that occur during the co-gelation of zirconium n-butoxide and copper nitrate are illustrated in Fig. 1(a) and (b). X-Ray Diffraction. The undoped zirconia sample was amorphous below 400◦ C, and crystallized to form the tetragonal phase at this temperature. At 600◦ C, a small peak near 28.2◦ (2θ) can be seen (Fig. 2(a)), indicating the formation of the monoclinic phase. As the temperature increases, the presence of monoclinic zirconia is confirmed by the appearance of a second, well-defined peak at 31.3◦ (2θ). When zirconia is doped with 1% mol copper (Fig. 2(b)), there is no significant change in the crystalline phases observed below 800◦ C. However, at this temperature, the relative abundance of the monoclinic phase is significantly higher than observed in the undoped sample, with the intensity of the characteristic peak at 28.2◦ (2θ) increases up to four times compared to that in the undoped material. This indicates that at high temperatures, copper doping promotes the formation of the monoclinic phase. No evidence of discrete crystalline copper compounds was observed. The Debye-Scherrer formula [8] was used to calculate the apparent crystallite size of the tetragonal phase, from the measured full-width at half-maximum of the 30.3◦ peak. Below 800◦ C, the crystallite size varied between 9 and 12 nm for both samples, but at 800◦ C, it had increased to 16 and 18 nm for the undoped and doped samples, respectively. In general, the crystallite size in the copper-doped samples was ca. 10% larger than in the undoped material. FTIR Spectra. The evolution of the FTIR spectra as a function of annealing temperatures is illustrated in Fig. 3. All samples exhibited bands at 450– 520 cm−1 , characteristics of Zr O bonds. The bands in the 1300–1400 cm−1 range are attributed to C H scissor modes, associated with organic residues which disappear when the materials were treated above 400◦ C. At 3450 cm−1 , a broad band is observed corresponding to O H stretching vibrations of molecular water (either free or H-bonded), which disappeared when the annealing temperature was increased. The band profile around 1630 cm−1 contains contributions from water (O H O bending mode) and nitrate species (O NO2
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Figure 1. Sol–gel reactions: (a) hydrolysis reaction and (b) condensation reaction.
antisymmetric stretch) [11], which also decrease in intensity with increasing temperature, as expected. UV-Vis Spectra. From the UV-Vis spectroscopic analysis, it was observed that the E g values decreased with increasing temperature. In Fig. 4, we observe that the bandgap for the undoped sample decreases from 4.17 at 200◦ C to 3.74 eV at 800◦ C. These values
are decreased significantly on doping, with a value of 3.25 eV being observed for the doped material after heating at 800◦ C. This behavior is due to the transformation of the electrical properties of the solid, from an insulator to a semiconducting material. Therefore, we expect that the powders obtained can be used as photocatalysts for the degradation of aromatics in waste water; this will be the subject of future investigations.
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Figure 4. Band gap (E g ) values for ZrO2 and Cu/ZrO2 as a function of annealing temperature.
4.
Figure 2. XRD patterns for: (a) pure ZrO2 and (b) 1% mol Cu doped ZrO2 . Both samples were annealed at 400, 600 or 800◦ C. Zm denotes peaks assigned to monoclinic zirconia.
Conclusions
The addition of copper in zirconia sol–gel processing promotes the formation of the monoclinic phase at higher temperatures; nevertheless, below 600◦ C the particle size of the tetragonal phase does not vary significantly in either the doped or undoped zirconia. The main effect of doping was observed in the band gap values, with E g decreasing with increasing temperature. When copper is co-gelled with zirconia, the lowest E g values are obtained. Since the copper-doped zirconia has a low E g value at 400◦ C, while still retaining the tetragonal structure, we expect that this sample will find potential applications as a photocatalyst. Acknowledgments We would like to acknowledge CONACyT for the grant given to M. Alvarez (No.171497). References
Figure 3. FTIR spectra of Cu/ZrO2 samples: (a) as-prepared, (b) 200, (c) 400, (d) 600, and (e) 800◦ C.
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